Reflux of demethenization columns

ABSTRACT

A process for the separation of the components of a gas mixture to be treated comprising methane, nitrogen and at least one hydrocarbon having at least two carbon atoms, or a mixture of these hydrocarbons, including introducing the gas mixture to be treated into a first distillation column, thereby producing, in the column bottom, a first liquid stream enriched in hydrocarbon having at least two carbon atoms and, at the column top, a first gas stream enriched in methane; and introducing the first liquid stream enriched in hydrocarbon having at least two carbon atoms resulting from stage a) into a second distillation column, thereby producing, at the top of this column, a second gas stream rich in methane and, in the bottom of this column, a second liquid stream comprising at least 85 mol % of the hydrocarbons having at least two carbon atoms initially present in the mixture to be treated.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of International PCT Application PCT/FR2016/052489, filed Sep. 29, 2016, which claims priority to French Patent Application No. FR 1560528, filed Nov. 3, 2015, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a process for the separation of the components of a gas mixture containing methane, nitrogen and heavier hydrocarbons than methane.

The present invention thus applies to processes for the denitrogenation of natural gas, with or without recovery of helium.

Natural gas is desirable for use as fuel intended to be used for heating buildings, in order to provide heat for industrial processes, for the production of electricity, for use as starting material for various synthetic processes for producing olefins, polymers and the like.

Natural gas occurs in numerous fields which are at a distance from the users of natural gas. Natural gas is typically composed of methane (C₁), ethane (C₂) and heavier compounds, such as hydrocarbons having at least three carbon atoms, such as propane, butane, and the like (C₃+). It can often be advantageous to separate the C₂ and C₃+ compounds from the natural gas in order to market them as separate coproducts.

This is because their value is generally greater than natural gas itself as they can be used directly for chemical processes (manufacture of ethylene from ethane, for example), as fuels (C₃/C₄ is a conventional fuel known as LPG) or for many other applications.

Another component often present in natural gas is nitrogen. The presence of nitrogen in natural gas can cause difficulties in observing the specifications for natural gas (typically minimum net calorific value to be observed). This is all the more true when the heavier hydrocarbons than methane (C₂ and C₃+ compounds) are removed as these have a higher net calorific value than methane; by removing them, the net calorific value is thus reduced, and it may then be necessary to increase it by the separation of nitrogen.

Consequently, a considerable effort has been devoted to the development of means for removing the nitrogen present in natural gas.

The natural gas deposits exploited increasingly contain nitrogen. This is explained in particular by the exhaustion and increased scarcity of fields which are sufficiently rich for no enriching treatment to be necessary before the marketing of the gas.

Frequently, these natural gas sources also contain helium. The latter can be made economic use of by carrying out a preconcentrating, before final treatment and liquefaction.

Unconventional resources, such as shale gases, also have the same problem: in order to make them marketable, it may prove to be necessary to increase their calorific value by means of a treatment which consists in denitrogenating the gas.

The most widely used method for separating nitrogen and heavier hydrocarbons than methane is “cryogenic separation”. A cryogenic process for the separation of nitrogen, more specifically a process employing a double column, is described in the patent application U.S. Pat. No. 4,778,498.

Natural gas denitrogenation units generally treat gases which originate directly from wells at a high pressure. After denitrogenation, the treated gas has to be sent back to the network, often at a pressure close to its entry pressure.

During the exploitation of natural gas deposits, numerous stages may be provided. A relatively conventional stage after the drying and the withdrawal of the impurities is the separation of the liquids associated with the natural gas (NGLs).

This stage can have many advantages but often it is a matter of making economic use of various “heavy” hydrocarbon products containing at least two carbon atoms (C₂, C₃, and the like, compounds) which are generally sold much more expensive than the natural gas product.

If the natural gas contains nitrogen, there is a risk of re-encountering a natural gas having an excessively low calorific value because of the low resulting content of C₂, C₃, and the like, compounds. It is thus typical to then have to separate the nitrogen from this gas in order to make it marketable.

A conventional solution is to treat the two problems independently.

A first unit carries out the separation of the NGLs (subsequently known as NGL unit), while a second unit separates the nitrogen from the natural gas (subsequently known as NRU). This solution exhibits the advantage of operational flexibility.

For example, if the NRU unit comprises a refrigeration cycle, the associated devices have a limited reliability, and a failure of a cycle compressor will result in the shutdown of the NRU but without resulting in the shutdown of the NGL.

Unfortunately, this shutdown cannot be lengthy in duration since it will then be necessary to send the production to the flare stack (because of its excessively low calorific value).

In addition, this scheme is limited in terms of effectiveness as all the gas is During a treatment in an NGL separation unit, a significant fraction (typically more than 10%) of the feed gas is condensed. During this condensation, methane is condensed with the heavier hydrocarbons (C₂+ and/or C₃+ compounds).

It is then typically necessary to use a column known as a demethanizer in order to reboil the methane and not to lose methane in the C₂+ and/or C₃+ products. If nitrogen is present, the latter will, on the other hand, be only very slightly condensed and will be re-encountered predominantly in the gas phase introduced into the demethanization column.

SUMMARY

The inventors of the present invention have thus developed a solution which makes it possible to solve the problem raised above while optimizing the energy costs, such as, for example, those related to the electrical consumption during the implementation of such processes.

A subject matter of the present invention is a process for the separation of the components of a gas mixture to be treated comprising methane, nitrogen and at least one hydrocarbon having at least two carbon atoms, or a mixture of these hydrocarbons, comprising the following stages:

a) introduction of the gas mixture to be treated into a first distillation column in order to create, in the column bottom, a first liquid stream enriched in hydrocarbon having at least two carbon atoms and, at the column top, a first gas stream enriched in methane;

b) introduction of the said first liquid stream enriched in hydrocarbon having at least two carbon atoms resulting from stage a) into a second distillation column in order to create, at the top of this column, a second gas stream rich in methane and, in the bottom of this column, a second liquid stream comprising at least 85 mol % of the hydrocarbons having at least two carbon atoms initially present in the mixture to be treated;

characterized in that a portion of the second gas stream rich in methane resulting from stage b) at the outlet of the top of the second distillation column is compressed to a pressure greater by 1 bar than the pressure of the second distillation column and then condensed in order to be introduced, for one portion, into the upper part of the first distillation column and, for the other portion, into the upper part of the second distillation column in order to carry out the reflux of these said distillation columns.

More particularly, a subject matter of the present invention relates to:

-   -   A process as defined above, characterized in that a portion of         the second gas stream is reduced in pressure in a turbine to a         pressure at least lower by 1 bar than the pressure of the         distillation column.     -   A process as defined above, characterized in that the nitrogen         content of said second gas stream is at least 1.5 times lower         than the nitrogen content of the first gas stream.     -   A process as defined above, additionally comprising stage c):         introduction of said first gas stream enriched in methane         resulting from stage a) into a denitrogenation unit in order to         separate the nitrogen from the other components of this gas         stream.     -   A process as defined above, characterized in that the second gas         stream resulting from stage b) is not treated by the         denitrogenation unit.     -   A process as defined above, characterized in that from 5 mol %         to 30 mol % of the methane initially present in the gas mixture         to be treated is comprised in the first liquid stream enriched         in hydrocarbon having at least two carbon atoms resulting from         stage a).     -   A process as defined above, characterized in that from 10 mol %         to 20 mol % of the methane initially present in the gas mixture         to be treated is comprised in the first liquid stream enriched         in hydrocarbon having at least two carbon atoms resulting from         stage a).     -   A process as defined above, characterized in that, prior to         stage a), it comprises the following stages:     -   at least partial condensation of said gas mixture to be treated         in order to obtain a two-phase mixture;     -   injection of the vapor phase from said two-phase mixture into         said first distillation column;     -   injection of the liquid phase from said two-phase mixture into         said second distillation column.     -   A process as defined above, characterized in that the gas         stream, extracted from the first distillation column in stage         a), comprises at most half of the amount of hydrocarbons having         more than two carbon atoms present in the feed gas.     -   A process as defined above, characterized in that the liquid,         extracted from said first distillation column during stage a),         comprises at least 90 mol % of the hydrocarbons having at least         two carbon atoms and preferably at least 95%.     -   A process as defined above, characterized in that said gas         mixture to be treated comprises 70 mol % of methane, at least 4         mol % of nitrogen and 2 mol % of hydrocarbons having at least         two carbon atoms.     -   A process as defined above, characterized in that, during stage         b), the liquid stream enriched in hydrocarbon having at least         two carbon atoms resulting from stage a) is introduced into said         second distillation column at a theoretical stage below the top         of this said second column.     -   A process as defined above, characterized in that said gas         stream resulting from stage b) is extracted directly from said         second distillation column at a pressure of greater than 20 bara         and comprises 95 mol % of methane.

Thus, the process which is a subject matter of the present invention makes it possible to take advantage of the fact that only the gas feed of the methanization column substantially contains nitrogen.

This is because the solution provided, in comparison with the known processes of the state of the art, is that of splitting in two the demethanization column conventionally used, one (that is to say, the first distillation column) producing a natural gas product poor in C₂+ and rich in nitrogen and the other (that is to say, the second distillation column) containing a natural gas product poor in C₂+ and denitrogenated.

The process of the invention makes it possible to separate a crude gas rich in C₂+ and in nitrogen (typically at least 1% of C₂+ and at least 2% of nitrogen). According to a specific embodiment, the process according to the invention typically comprises the following stages:

-   -   Pretreatment of the crude gas to be treated (separation of the         water, CO₂, methanol, heavy hydrocarbons, for example).     -   Cooling of the crude gas to a first subambient temperature         (typically between −30° C. and −70° C.), making it possible to         obtain a cooled two-phase stream.     -   Separation of the cooled two-phase stream into a first gas and a         first liquid.     -   Reduction in pressure of at least a portion of the first liquid         in order to inject it into a lower demethanization column, known         above as the second distillation column. Alternatively, a gas         stream containing less nitrogen than the initial gas stream to         be treated is withdrawn at a lower level than that of the top of         the first demethanization column in order to be injected into a         lower demethanization column, known above as the second         distillation column.     -   Reduction in pressure of at least a portion of the gas in a         turbine and introducing it, after reduction in pressure, into         the bottom of an upper demethanization column, known above as         first distillation column.     -   Condensation of at least a portion of the gas in order to use it         as reflux in the upper demethanization column.     -   Introduction of the liquid from the upper demethanization column         into the lower demethanization column at at least one         theoretical stage below the column top.     -   Obtaining, at the top of the upper demethanization column, a gas         poor in C₂+ and rich in nitrogen (richer than the crude gas).     -   Obtaining, at the top of the lower demethanization column, a gas         poor in C₂+ and poor in nitrogen (typically containing at least         two times less nitrogen than the top of the upper column and         less than a quarter of nitrogen with respect to the amount of         nitrogen of the top of the “upper” column).

This also makes it possible to considerably simplify the sheet-metal working of the demethanization columns conventionally employed in the known processes of the state of the art.

This is because, typically, the column top is much wider than the column bottom, which presents mechanical constraints and thus additional costs. The separation of the columns makes it possible to escape this constraint.

In order to deplete the top of the lower demethanization column, an additional reflux is provided. This reflux should, if possible, be very poor in nitrogen. Several means are possible for providing this reflux:

-   -   Use of a dedicated condenser, for example with liquid methane at         a lower pressure than the column top. This liquid methane can be         produced by the downstream part of the process (NRU).     -   Use of the recompressed and recondensed top gas from the lower         demethanization column.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail with reference to the FIGURE, which illustrates a process according to the invention.

-   -   The FIGURES illustrates a schematic representation of one         embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the FIGURE, a flow 1 of natural gas pretreated beforehand (typically having undergone a separation of a portion of at least one of the following constituents: water, CO₂, methanol, sulfur compounds, very heavy hydrocarbons, that is to say having more than six or seven carbon atoms (such as C₈+ compounds, for example)) comprising at least 30 mol % of methane, at least 0.1 mol % of heavier hydrocarbons than methane (that is to say comprising at least two carbon atoms) and between at least 4 mol % et 50 mol %, indeed even 80 mol %, of nitrogen is introduced into a system 2 which makes possible an at least partial condensation of said flow 1.

The pressure of this flow 1 is between 20 bara (bar absolute) and 100 bara (typically between 30 and 70 bara) and the temperature is close to ambient temperature, for example between 0° C. and 60° C.

The system 2 is, for example, a heat exchanger. The mixture 3 exiting from this system 2 is in a two-phase (gas and liquid) state. This mixture 3 is introduced into a phase separator 4.

The operating pressure is between 20 and 100 bara, typically between 30 and 70 bara. The temperature of this separator is between −100° C. and 0° C., typically between −80° C. and −20° C.

At least a portion 8′ of the gas phase 8 resulting from the separator 4 is reduced in pressure by means of a turbine 9. The flow resulting from the turbine 9 is introduced into a first distillation column 7 at a stage 10 located in the lower part of said column 7.

The liquid phase 5 resulting from the separator 4 is reduced in pressure through a valve 6 and is then injected, at a pressure of between 10 bara and 40 bara and a temperature, for example, of between −110° C. and −30° C., into a demethanization column 7′, hereinafter also known as second distillation column. This liquid phase 5 is introduced at a theoretical stage 10′ below the top of said column 7′.

A liquid flow 12 of heavier hydrocarbons than methane is recovered in the lowest part 16 (in the bottom) of the column 7′.

A reboiler 11 is placed at a level which makes it possible to reboil the bottom liquid from the column 7′ in order to reheat a portion of the liquid from said column with the aim of adjusting the maximum threshold for methane present in the flow 12 of heavy hydrocarbons.

At least 50 mol % (typically at least 85 mol %) of the heavy hydrocarbons present in the gas mixture 1 to be treated are recovered in this flow 12. Preferably, at least 90% are recovered.

Preferably, the liquid flow 12 of hydrocarbons does not contain more than 1 mol % of methane.

A heat exchanger 13 can be installed in order to reheat the bottom part of the column 7′ (bottom part=below the introduction of the liquid originating from the separator 4). This exchanger is fed with the gas feed stream 1. This reheating improves the balance between search for maximum yield and purity of the outlet stream from this second “demethanization” distillation column 7′.

At the top 14 of the first distillation column 7 (top=highest outlet of the column), a gas flow 15 enriched in methane, typically containing less than 0.5 mol % of hydrocarbons having more than two carbon atoms (containing at most half the amount of heavy hydrocarbons—having more than 2 carbon atoms—present in the feed gas), is extracted. The temperature of the gas stream 15 is less than −80° C.

In the bottom 38 of the first distillation column 7, a liquid stream 39 is extracted in order to be introduced into said second distillation column 7′ at a stage 10″ substantially at the same level as that 10′ where the liquid phase 5 resulting from the phase separator 4 is introduced.

This liquid stream 39 resulting from the first distillation column 7 is depleted in nitrogen (typically containing less than 10%, preferably less than 5%), just like the liquid phase 5 resulting from the separator 4. Gas depleted in nitrogen is understood to mean a gas stream having a nitrogen content which is less than half the nitrogen content of the initial gas stream 1 to be treated and preferably less than a quarter of this content. The result of this is that very little nitrogen is introduced into the second distillation column 7′.

Consequently, the gas stream which will be extracted from this second distillation column will not have to be introduced into an NRU unit, which will greatly lighten the burden on this NRU unit which will have to treat the gas stream 15 resulting from the first distillation column 7.

Typically, between 10% and 20% of the methane initially present in the gas stream 1 to be treated will be re-encountered in this liquid stream 39 introduced into the second distillation column 7′ and thus will not have to be introduced into an NRU unit.

Demethanization column is understood to mean a distillation column intended to produce at least two streams which are different in composition starting from a feed stream to be treated according to the process of the present invention.

The at least two streams are as follows: one, at the column top, gaseous, depleted in hydrocarbons having at least two carbon atoms, that is to say comprising less than half of the “heavy” hydrocarbons present in the feed gas (ethane, propane, butane, and the like), and the other, in the column bottom, in the liquid form, depleted in methane present in the feed stream to be treated.

The molar concentrations of the different components of the streams of the different stages of the process as illustrated according to the FIGURE are shown in the table below.

It may then be observed that the stream 39 is a liquid comprising predominately methane and, to a minor extent, ethane and propane and contains virtually no nitrogen.

° 1 39 15 12 15′ Methane 88.6% 93.5% 93.9%  1.4% 97.4% Ethane  4.8%  5.7%  0.4% 69.9%  0.4% Propane  1.3%  0.2%  0.0% 19.7%  0.0% Isobutane  0.2%  0.0%  0.0%  2.8%  0.0% n-Butane  0.3%  0.0%  0.0%  4.5%  0.0% Isopentane  0.1%  0.0%  0.0%  1.0%  0.0% n-Pentane  0.0%  0.0%  0.0%  0.6%  0.0% Helium  0.1%  0.0%  0.2%  0.0%  0.1% Nitrogen  4.5%  0.6%  5.5%  0.0%  2.1%

Demethanization unit is understood to mean any system comprising at least one distillation column for enriching the top gas in methane and depleting the bottom liquid in methane.

The gas stream 15′ is extracted at the top 14′ of the second distillation column 7′ at a temperature of between −80° C. and −120° C. and at a pressure of greater than 10 bara (typically of between 15 bara and 30 bara). A portion of this gas stream 15′ is introduced into a heat exchanger 17 in order to be reheated to a temperature of between −50° C. and −110° C., is then introduced into a turbine 43 before rejoining a stream enriched in methane 30 at the outlet of a denitrogenation unit A and is produced at the end of the process as natural gas. The pressure of this natural gas produced is, for example, of between 15 bara and 30 bara (before recompression) and the temperature is greater than 0° C. after reheating in the exchanger 2.

It is possible to condense a gas enriched in methane under pressure in order to improve the performance qualities.

This condensation is carried out by virtue of a heat exchanger 17 fed with a portion 8″ of the gas flow 8 resulting from the phase separator 4 and with a portion 44 of the gas stream 15′ extracted from the top 14′ of the second distillation column 7′.

Before being introduced into the heat exchanger 17, this gas stream 44 is compressed using a compressor 46, ideally a cold compressor, that is to say the temperature of which is less than 0° C. It is this compressed stream 45 which feeds the heat exchanger 17. The power of the compressor 46 can advantageously originate from the turbine 43, which makes it possible to optimize the compression.

The pressure of the stream 44 is increased by a few bara only in order for the stream 45 to be able to be recondensed countercurrentwise. The stream or streams 18 (18 b and 18 b) which has (have) been cooled in the exchanger 17 is (are) reduced in pressure by means, for example, of at least one valve 19 (19 a, 19 b) and is (are) then introduced into a top part (top part=above the feed 10 exiting from the turbine 9) of the column 7.

The reflux of the second distillation column 7′ is provided, in the same way as for the reflux of the first distillation column 7, by the introduction, into its upper part 41, of at least one stream 18 c which has been cooled in the exchanger 17 and reduced in pressure by means, for example, of at least one valve 19 c.

These reflux stages are necessary in order to feed the two columns 7 and 7′ with cold liquid poor in C₂+.

The use of a portion 44 of the gas stream 15′ enriched in methane and not containing nitrogen makes it possible not to have to recycle outputs in order to increase the yield of C₂+ products.

The gas stream enriched in methane 15 resulting from the top 14 of the distillation column 7 is partially condensed by means, for example, of a heat exchanger 21. There emerges, at the outlet of this exchanger 21, a two-phase (gas/liquid) stream 22 (comprising from 20 mol % to 80 mol % of gas).

The temperature of the stream 15 is kept below −80° C. (or even below −100° C.) and said stream 15 is directly in the heat exchanger 21 in order to obtain the stream 22.

The stream 22 is subsequently sent to a denitrogenation system A.

In the denitrogenation system A, the two-phase stream 22 is, after an optional reduction in pressure in a valve or a turbine 23, introduced into a phase separator 25.

The liquid phase 29 resulting from the phase separator 25 is, after an optional reduction in pressure, reheated through the heat exchangers 27, then 21 and finally 2 in order to rejoin the outlet stream 30 of gas rich in methane produced at the process outlet. The outlet stream 30 contains less than 5 mol % of nitrogen.

The gas phase 26 resulting from the separator 25 is partially condensed in a heat exchanger 27 and then reduced in pressure at the outlet of said exchanger 27 by means of a turbine or of a valve 28 before being introduced into a distillation column 31.

The distillation column 31 is a column for stripping nitrogen, the aim of which is to separate the nitrogen from the liquid enriched in methane at the outlet, also known as denitrogenation column. The liquid enriched in methane comprises less than 5 mol % of nitrogen. In this instance, a distillation column joined to a reboiler 32 but not having available an associated condenser system is concerned.

A stream 33 very rich in methane in the liquid form is extracted at the bottom of the column 31, at a temperature of less than −100° C., preferably of less than −110° C. This stream 33 contains less than 5 mol % of nitrogen, preferably less than 4%. The liquid stream 33 is subsequently mixed with the liquid phase 29 resulting from the phase separator 25 and follows the same path as far as the outlet streams 30, 30′.

A gas flow 36 rich in nitrogen, at a temperature of less than −110° C., is produced at the top 35 of the column 31. Said flow 36 rich in nitrogen comprises at least 20 mol % of nitrogen.

The flow rich in nitrogen 36 is reheated through the successive exchangers 27, 21, then 2. It can also be one and the same exchanger, according to a specific embodiment of the invention. Moreover, according to another specific embodiment of the invention, more than three exchangers can be employed.

This then results in a stream 37, at a temperature close to ambient temperature (greater than −10° C. typically and less than 50° C.), sent to an additional denitrogenation system B.

The aim of the denitrogenation system B is to produce a gas stream even richer in nitrogen than the stream 37. This system B can, for example, include at least one separator and one denitrogenation column.

If the specification for nitrogen at the outlet of the system B is strict (<100 ppm typically), it may prove to be necessary to add, to the system B, a cycle compressor, for example a nitrogen or methane compressor, in order to contribute the reflux necessary in order to obtain the nitrogen purity at the top of the denitrogenation column of the system B.

A specific NRU unit has been described in this FIGURE but the process which is a subject matter of the present invention applies to any type of NRU unit downstream of an “NGL” unit.

More advantageously still, the process which is a subject matter of the present invention makes it possible to achieve savings in terms of electricity consumption, for example. This is because only a portion of the methane included in the gas to be treated is sent to an NRU unit, as the other portion which occurs in the bottom of the first distillation column in the liquid form does not contain nitrogen, with the result that the NRU unit downstream of the NGL unit is much less burdened.

This represents a saving in electricity consumption of the order of 10% to 30%.

Advantageously, the process which is a subject matter of the present invention makes it possible for the device which will employ it to treat a reduced output as the recycle originating from the outside necessary in the normal processes of the state of the art is dispensed with: only the output of the gas mixture to be treated 1 is used. This can represent a saving in gas normally used for the reflux of the distillation columns of the order of 10%.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 

1.-13. (canceled)
 14. A process for the separation of the components of a gas mixture to be treated comprising methane, nitrogen and at least one hydrocarbon having at least two carbon atoms, or a mixture of these hydrocarbons, comprising the following stages: a) introducing the gas mixture to be treated into a first distillation column, thereby producing, in the column bottom, a first liquid stream enriched in hydrocarbon having at least two carbon atoms and, at the column top, a first gas stream enriched in methane; b) introducing the said first liquid stream enriched in hydrocarbon having at least two carbon atoms resulting from stage a) into a second distillation column, thereby producing, at the top of this column, a second gas stream rich in methane and, in the bottom of this column, a second liquid stream comprising at least 85 mol % of the hydrocarbons having at least two carbon atoms initially present in the mixture to be treated; wherein a portion of the second gas stream rich in methane resulting from stage b) at the outlet of the top of the second distillation column is compressed to a pressure at least greater by 1 bar than the pressure of said second distillation column and then condensed in order to be introduced, for one portion, into the upper part of the first distillation column and, for the other portion, into the upper part of the second distillation column in order to carry out the reflux of said distillation columns.
 15. The process as claimed in claim 14, wherein a portion of the second gas stream is reduced in pressure in a turbine to a pressure at least lower by 1 bar than the pressure of said second distillation column.
 16. The process as claimed claim 14, wherein the nitrogen content of said second gas stream is at least 1.5 times lower than the nitrogen content of the first gas stream.
 17. The process as claimed in claim 14, further comprising stage c): introduction of said first gas stream enriched in methane resulting from stage a) into a denitrogenation unit in order to separate the nitrogen.
 18. The process as claimed in claim 17, wherein the second gas stream resulting from stage b) is not treated by the denitrogenation unit.
 19. The process as claimed in claim 18, wherein from 5 mol % to 30 mol % of the methane initially present in the gas mixture to be treated is comprised in the first liquid stream enriched in hydrocarbon having at least two carbon atoms resulting from stage a).
 20. The process as claimed in claim 14, wherein from 10 mol % to 20 mol % of the methane initially present in the gas mixture to be treated is comprised in the first liquid stream) enriched in hydrocarbon having at least two carbon atoms resulting from stage a).
 21. The process as claimed in claim 14, further comprising, prior to stage a), the following stages: at least partially condensing said gas mixture to be treated in order to obtain a two-phase mixture; injecting the vapor phase from said two-phase mixture into said first distillation column; injecting the liquid phase from said two-phase mixture into said second distillation column.
 22. The process as claimed in claim 14, wherein the gas stream, extracted from the first distillation column in stage a), comprises at most half of the amount of hydrocarbons having more than two carbon atoms present in the feed gas.
 23. The process as claimed in claim 14, wherein the liquid, extracted from said first distillation column during stage a), comprises at least 90 mol % of the hydrocarbons having at least two carbon atoms and preferably at least 95%.
 24. The process as claimed in claim 14, wherein said gas mixture to be treated comprises 70 mol % of methane, at least 4 mol % of nitrogen and 2 mol % of hydrocarbons having at least two carbon atoms.
 25. The process as claimed in claim 14, wherein, during stage b), the liquid stream enriched in hydrocarbon having at least two carbon atoms resulting from stage a) is introduced into said second distillation column at a theoretical stage below the top of this said second column.
 26. The process as claimed in claim 14, wherein said gas stream resulting from stage b) is extracted directly from said second distillation column) at a pressure of greater than 20 bara and comprises 95 mol % of methane. 